The screening and diagnosis of osteoporosis is becoming an increasingly important health care priority. This is because osteoporosis is generally a disease of aging and thus is becoming more widespread with the ever-increasing life span of the world's population. Osteoporosis often results in painful and debilitating fractures of the spine and hip.
Early screening and diagnosis of osteoporosis can result in the afflicted person receiving therapy which may prevent such debilitating fractures. In screening for osteoporosis it is recommended that a physician obtain both a bone density ("BMD") scan of the patient's skeleton, particularly the spine, and lateral x-rays of the spine for evidence of fracture. The combination of low spinal bone density and vertebral fracture is generally accepted as diagnostic of osteoporosis. With today's bone densitometers, the determination of spinal BMD is relatively straightforward. Determining whether there is evidence of vertebral fracture by subjectively evaluating a lateral radiograph tends to be a more problematic task. Hedlund, L. R. and J. C. Gallagher, "Vertebral morphometry in diagnosis of spinal fractures", Bone and Mineral, vol. 5, 59-67 (1988). Even in patients with established osteoporotic fractures it is difficult to reliably indicate the nature and extent of vertebral deformation because there are not consistent criteria for evaluation.
This is because in the early stages of osteoporosis, the subjective examination of a lateral radiograph of the vertebral column is often ineffective in diagnosing vertebral deformation or fracture. This is particularly true if there is no previous radiological record available for comparison. Vertebral deformities or fractures are classified into crush deformities, wedge deformities and end-plate deformities. In the latter type of deformity, one has biconcavity and hypertrophy types of deformities. An early crush fracture often will reveal no sharply defined fracture line and thus it is virtually impossible to subjectively distinguish between a vertebral body with a early crush fracture and a normal vertebral body.
Such diagnosis is more effectively made by conducting what is known as vertebral morphometric analysis. Vertebral morphometric analysis is a detailed examination of the shape and dimensions of vertebral bodies as determined from a lateral radiograph of the spine. Using this method, the clinician obtains an analog x-ray image of the patient's vertebral column using a conventional x-ray machine. The image is printed onto a fixed media such as an x-ray radiographic film print. The print is made to a specific scale relative to the patient's spine, i.e. one-to-one, or a specifically reduced or expanded scale. Then the clinician manually measures the size of the vertebra by using a straight edge ruler and pencil. Using this technique, the clinician actually draws on the film to outline the vertebra, selects the appropriate fiducial points and then measures with the ruler the distance between the fiducial points. The more common measurements made in a vertebral morphometric study are the anterior (H.sub.a), middle (H.sub.m) and posterior height (H.sub.p) of the vertebra. The heights themselves, compared to normal or adjacent vertebra or to a prior study, or the ratio of these heights, one to the other, can be diagnostic of fracture. For example, a decrease in H.sub.a by 15 to 20%, which can be anywhere from 1.8 to 7 mm depending on the size and age of the individual, is considered by some physicians to indicate fracture. See e.g. Smith-Bindman, R. et al., "A Comparison of Morphometric Definitions of Vertebral Fracture", J. Bone and Min. Res., Vol. 6 No. 7, 25-34 (1991).
While the manual technique described above is still widely used, computerized techniques have also become available. One computerized technique requires an analog lateral radiograph which is then digitized. Once digitized, the radiographic image is analyzed using a software program which provides measuring tools but still requires the clinician to manually select the fiducial points for measurement. Nelson, D., et al., Measurement of vertebral Area on Spine X-rays in Osteoporosis: Reliability of Digitizing Techniques, J. Bone and Mineral Research, Vol. 5, No. 7, 707-715, (1990).
More recently, an apparatus and method have been described comprising a digital x-ray machine and software which automatically analyzes the vertebral morphometry. U.S. Pat. No. 5,228,068 issued to Lunar Corporation, incorporated herein by reference, describes such a device. Unlike a conventional x-ray machine which utilizes a cone-shaped x-ray beam, the aforementioned digital x-ray machine utilizes a scanning fan beam. One embodiment of the invention described and claimed in the aforementioned patent is the use of a dual energy x-ray and software which permits not only the analysis of the morphometry of the vertebra but also a determination of bone density of the vertebra.
Regardless of the technique used, distortion of the image can occur leading to erroneous results and misdiagnosis. For example, the conventional x-ray machine, because of the cone shaped x-ray beam, produces a radiographic image which is typically 10-15% larger than life-size, and the magnification is variable depending on the location of the object relative to the plane of the radiograph. Differential magnification occurs due to spine curvature. Distortions of the spine are particularly acute for cone beam exposures at the edges of the cone beam where the ray is most angled. Distortion also occurs because of scattered radiation which causes blurring of the edges thus making it difficult for both the operator and the software to easily detect the edges of the vertebra for measurement.
A scanning fan beam can be used to avoid some of the distortion problems which occur with the conventional cone beam. Blurring because of scattered radiation is greatly reduced in a scanning system. Also, because of the geometry of the beam, magnification in the direction of the scan is greatly reduced. However, distortion can still occur particularly at the edges of the beam where the rays are more angled. Furthermore, as the alignment of the spine is often curvilinear, the desired measurement may not be in the direction of least distortion. Also, the alignment of the fan beam with the detector is more critical with a scanning system than with a conventional x-ray machine and slight misalignment can result in a distortion of the image.
Distortion of the image can lead to misregistration of the software or inconsistent in operator identification of fiducial points. For example, a height measurement may vary if the operator or the computer software is not consistent in selecting the fiducial points. These variations make it difficult to be certain that the measurements are accurate and truly represent the actual morphometry of the patient's vertebrae. Significantly, distortion induced errors in morphometric measurements can not only cause the misdiagnosis of a normal vertebra as having fracture (false positive), but can cause fractured vertebrae to appear normal (false negative).
Both subjective analysis of morphometry by an operator and computer software analysis of a vertebrae may be influenced by a rotation of the vertebra within the image plane and by its relative orientation with respect to its neighbors.
Prior art phantoms exist which purport to allow checking of image magnification and angle settings. One such phantom which is marketed by Cone Instruments and named QA RADIOGRAPHIC PHANTOM has a representative "normal" vertebra. Another phantom is manufactured by Hologic, Inc. of Waltham, Mass. and called the X-CALIBER. However, none of these phantoms permit easy verification that the devices or the software of the newer digital morphometry systems are working properly. None of these phantoms present the types of pathologies that the vertebral morphometry systems are designed to detect and none of these prior art phantoms are suitable for use in the training of techniques required for analyzing vertebral morphometry.